This invention relates to papermakers' fabrics and especially to papermaking fabrics for the forming section of a papermaking machine.
In the conventional papermaking process, a water slurry or suspension of cellulose fibers, known as the paper "stock", is fed onto the top of the upper run of a traveling endless forming belt. The forming belt provides a papermaking surface and serves as a filter element to separate the aqueous medium from the cellulosic fibers by providing for the drainage of the aqueous medium through its mesh openings, also known as drainage holes, by vacuum means or the like located on the drainage side of the fabric to form the paper web. The bottom side of the forming belt contacts the papermaking machine elements, and thus must be resistant to abrasion for long fabric life.
After leaving the forming medium, the somewhat self-supporting paper web is transferred to the press section of the machine and onto a pres felt, where still more of its water content is removed by passing it through a series of pressure nips formed by cooperating press rolls, these press rolls serving to compact the web as well.
Subsequently, the paper web is transferred to a dryer section where it is passed about and held in heat transfer relation with a series of heated, generally cylindrical rolls to remove still further amounts of water therefrom.
Over the years, papermakers have sought improvements in the forming fabric, not only with respect to the operating life of the fabric, but also with respect to the quality of the paper sheet produced on it. Triple layer fabrics were introduced for this purpose. The triple layer fabric generally includes two distinct fabrics. The top fabric has a top surface designed specifically for papermaking to achieve the best possible sheet quality and machine efficiency. This top fabric is manufactured as an integral part of a woven structure with a completely separate bottom fabric designed specifically for mechanical stability and enhanced fabric life. The purpose of triple layer fabric development is to eliminate the compromises between optimizing papermaking surface or fabric life which exist with both single and double layer forming fabrics. With a triple layer fabric, papermakers can produce a top quality paper sheet at reduced cost, without sacrificing the wear characteristics of the papermaking fabric.
To produce a fine top quality paper sheet, a papermakers' fabric with fine sheet side yarns in the top fabric layer will be chosen. A fine uniform top fabric layer in the papermaking fabric will give a more uniform initial fiber mat than a coarse, nonuniform top fabric layer. This degree of fineness and uniformity influences subsequent layers of fiber as the sheet is formed, and eventually, the paper sheet produced. Triple layer fabrics of this design provide a high open area and fiber support to produce premium quality and high drainage capacity.
While triple layer fabrics with fine sheet side yarns have demonstrated success in numerous applications, other applications exist where somewhat coarser triple layer designs are preferred. A coarser triple layer fabric can be made using larger machine direction yarns on the sheet side. However, the use of larger warp yarns has been found to offer no advantage. Alternatively, a coarser triple layer fabric can be made using the same fine triple layer machine direction yarns but using coarser sheet side cross machine direction yarns.
For example, a triple layer fabric having 73 epi of 0.16 mm machine direction yarns, 80 ppi of 0.13 cross machine direction yarns and a plain weave top has an open area of 31.9%. Similarly, a fabric having 73 epi of 0.13 machine direction yarns and 80 ppi of 0.16 mm cross machine direction yarns has the same open area. Theoretically, then, the same fabric is produced.
The use of larger cross machine direction yarns instead of the larger machine direction yarns produces useful fabric properties. Edge curl of the fabric is controlled to a degree when larger cross machine direction yarns are used. Other benefits include improved warp stacking uniformity and paper making characteristics. The wire mark imparted to the paper made on a fabric with larger cross machine direction yarns is more evident, and therefore the paper inferior to that made on fabric with larger machine direction yarns using conventional stitching. This fact is especially true at higher cross machine direction yarn counts. An inferior paper sheet can be explained by the reduced drainage capacity of the fabric due to a blockage of holes at each sheet side stitch interlace.
The blockage can be seen with reference to the fabrics described above and FIGS. 1 and 2 showing prior art fabrics 10 and 15 in which the machine direction yarns are designated by reference number 11, cross machine direction yarns by reference number 13 and stitch yarns by reference number 14. As shown in FIG. 1, at 80 ppi with a 0.16 mm sheet side cross machine direction yarn 13 and a 0.13 mm stitch yarn 14, the stitch blocks approximately 83% of two holes at each interlace. Meanwhile, at 80 ppi with a 0.11 mm sheet side cross machine direction yarn 13 and a 0.13 mm stitch yarn 14, the stitch blocks only 63% of two adjacent holes, as shown in FIG. 2. The interrelation between hole blockage on the top picks per inch and sheet side fill yarn sizes with a 0.13 mm stitch is shown in FIG. 3.
Blockage of this type can cause a wire mark, on the forming paper sheet, which degrades the quality of the paper produced. The term "wire mark" is used to explain the micro or finer levels of density difference, often caused by the structure of the forming fabric on which the sheet is produced.
There has long been a need to produce a papermakers' fabric that combines good drainage ability with an optimal paper sheet surface. This is especially true when the application calls for a coarse triple layer fabric.
It is therefore an object of the present invention to prepare a papermaking fabric for the forming section of the papermaking machine.
Another object of the present invention is to provide a papermaking fabric that combines good drainage capability with an optimal paper sheet surface in a coarse triple layer fabric.
A further object of the present invention is to provide a papermaking fabric with good wear life and abrasion resistance that produces a paper sheet with optimal printing properties.
It is a further object of the present invention to allow flexibility in the selection of yarn dimensions and still retain optimum drainage capacity of the papermakers' fabric.